Electrolytic hygrometer cells and method of using same

ABSTRACT

AN IMPROVED METHOD OF MEASUREMENT OF THE MOISTURE CONTENT OF A GAS BY PASSING THE GAS THROUGH AN IMPROVED FORM OF ELECTROLYTIC HYGROMETER CELL WHEREIN THE AREA OF CONTACT OF THE GAS WITH THE DETECTOR ELEMENT OF THE CELL PROGRESSIVELY DECREASES IN SIZE FROM THE INLET TO THE OUTLET IN THE DIRECTION OF FLOW AND AN IMPROVED FORM OF DETECTING ELEMENT FOR SUCH A CELL WHEREIN THE INTERIDIGITATED ELECTRODE PATTERN IS EMBEDDED IN THE SURFACE OF A NONCONDUCTIVE SUBSTRATE.

Odi. 3, 1972 F BENNETT ETAL 3,696,007

ELECTROLYTIC HYGROMETER CELLS AND METHOD OF USING SAME Filed Feb. 3, 1970 2 Sheets-Sheet 8 United States Patent O 3,696,007 ELECTROLYTIC HYGROMETER CELLS AND METHD F USING SAME Frank Bennett, Croftways, Long St., Williton, Taunton,

Somerset, England, and Ivor .lohn Conibear, Millards Hill House, lnstow, ideford, North Devon, England Filed Feb. 3, 1970, Ser. No. 8,216 lut. Cl. G01n 27/46 U.S. Cl. 20d- 1 T 16 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the measurement of the moisture content of a gas, more particularly, but not exclusively to the measurement of relatively low moisture levels such as pertain in nuclear reactor gas coolants, and to an improved form of electrolytic hygrometer cell for such measurement, and an improved form of detector element for such cells.

Electrolytic hygrometer cells conventionally comprise platinum wire electrodes internally or externally wound on a former and sheathed in a tube which is internally coated with a phosphoric acid lilm as an hygroscoplc electrolyte. However, such cells are subject to difficulties in operation, and they are not readily suited to mass production and are accordingly expensive.

It has been proposed that the diiculties of manufacture, and other disadvantages, of the conventional wire wound cell form be avoided by the provision of a cell with a detector element in the form of printed circuit electrodes supported on an insulating substrate having a hygroscopic electrolyte lm coating, and housed in a generally flat container. However, it has been found that the production of such a detector element is extremely difficult, with materials appropriate to electrolytic hygrometry. One form of such flat cell is described in British Pat. No. 941,436 wherein the detector electrodes comprise two interleaved sets of conductors of platinum foil attached, as by fusing, to a glass substrate, the desired geometry being attained by printing techniques. The cell has a central inlet port from which the gas liows outwardly in contact with the hygroscopic electrolyte coating of the substrate and electrodes and then flows from the outer edges of the detector element to an outlet port.

With such form of dat cell the moisture content of a gas introduced into the cell is at a maximum in the inlet region where the electrode surface area is at a minimum so that rapid electrode destruction can arise from the resultant massive current densities obtaining in the region surrounding the inlet port. Moreover the electrodes themselves being formed by printed circuit techniques stand proud of the adjacent surface of the glass substrate are not only vulnerable to mechanical damage but also aggravate the current density problem in the region of the inlet port. This arises because the material of the hygroscopic electrolytic coating is attracted by surface tension into the spaces between adjacent conductors and electrolysis tends to be concentrated between the narrow confronting edges of the conductors rather than the relatively wider outer surfaces of the electrodes.

The present invention overcomes the drawbacks of known forms of flat hygrometer cells partly by an improved method of operating the cell and partly by the provision of an improved form of cell.

According to one aspect of the invention there is provided a method of measuring the moisture content of a gas by causing the gas to low through an electrolytic hygrometer cell to contact hygroscopic electrolyte material disposed between electrodes within such cell and measuring current ow between such electrodes, said method being characterised in that the gas is caused to flow in contact with said material along a flow path of which the area in contact with said material progressively diminishes in size along the length thereof in the direction of liow.

According to another aspect of the invention there is provided an electrolytic hygrometer cell for measuring the moisture content of a gas, said cell comprising a gastight hollow housing, a detector element in the form of an electrically non-conductive substrate having at least two electrode structures embedded in one face thereof and with a coating of hygroscopic electrolyte material extending over said face disposed in the interior of said housing with said face in spaced relation to adjacent interior surfaces of said housing so as to permit gas to contact said coating, a gas inlet port in the periphery of said housing disposed to admit gas into the interior of said housing said gas inlet port being directed substantially tangent with respect to said housing, a gas outlet port in the center portion of said housing spaced from said gas inlet port, said inlet and outlet ports, said interior surfaces of said housing and said coated face of said substrate defining between them a How passage for gas of which the area in contact with said coating progressively diminishes from said inlet port to said outlet port, and terminals extending from said electrode structures to the exterior of said housing.

According to yet another aspect of the invention there is provided a detector element for an electrolytic hygrometer cell said element comprising a substrate of electrically non-conductive material having two electrode structures embedded in a major face thereof and a coating of hygroscopic electrolyte material in contact with the outer surface of both embedded electrode structures and the surface of the substrate intervening between adjacent parts of the respective electrode structures.

The various features and advantages of the invention will be apparent from the following description of an exemplary embodiment thereof taken in conjunction with the accompanying drawings in which:

FIG. l is a perspective view of a hygrometer cell embodying the invention in fully assembled form,

FIG. 2 is a plan view of a detector element of the cell of FlG. 1, and

FIGS. 3a to 3i show the individual members which go to make up the assembly of FIG. l.

The cell 1 shown in FIG. l has an inlet connection 2 and an outlet connection 3 and a detector element having a coated electrode surface which is connected, within the cell, to two electrode terminal posts 4 and 5. The cell comprises a hollow housing made up of a top plate 6 and a base plate 7 with a series of annular elements, described later in relation to FIG. 3, disposed between them, the whole being bolted together by six bolts 8, the heads of which abut the top plate 6 and the opposite ends of which engage in threaded bores in the base plate 7.

A preferred form of the detector element of the cell of FIG. 1 is shown to a larger scale in FIG. 2. It comprises an annealed heat-resistant glass substrate 4d in the form of a circular disc having embedded in its upper face two interdigitated patterns of electrodes, one of which is connected to an outer peripheral contact area 32 and the other of which is connected to a circular central electrode area 37. Each electrode structure consists of four discrete groups of arcuate electrode members forming four segments 41, 42, 43, 44 of a circular pattern. In each segment the arcuate members of one electrode structure are connected in parallel along one side of the segment by a common radially extending conductor member 45 leading to the central contact area, and the arcuate members of the other electrode structure are connected in parallel along the opposite side of the segment by a common contact member 46 leading to the outer peripheral contact area 32.

Such a detector element is preferably made by etching the glass substrate 40 to form a pattern of grooves corresponding to the desired interdigitated pattern of the electrode structures, filling the groove pattern with a platinizing paste which upon firing forms `a coherent mass of metallic platinium, firing the substrate, surface lapping the fired substrate to remove any platinum from the glass lands between adjacent conductor members of the two electrode patterns and so that these lands are lightly matt n their upper surfaces, and finally coating the substrate uniformly with a thin film of hygroscopic electrolytic material such for example as a H3PO4 ilm over the area occupied by the four segments leaving the conductor areas 32 and 37 free of coating.

The form and arrangement of the various members making up the housing of FIG. l can best be seen from FIGS. 3(11) to 3(1') which will now be described commencing from the base of the housing when assembled.

The base plate 7 (FIG. 3(a)) comprises a solid metal disc having formed in its upper surface six internally threaded holes 9, 10, 11, 12, 13, 14 to receive the lower ends of six bolts 8, (FIG. 3(i)) two internally threaded locating holes 16 and 17 to receive the end of two clamping screws 18 (FIG. 3(h)) and respective inlet and outlet bores 19 and 20 communicating through the material 0f the disc with the inlet and outlet connections 2 and 3. Positioned on the upper surface of base plate 7 is an annular insulating gasket 22 (FIGS. 3( b)) made of PTFE and provided with holes registering with the various holes and bores in the base plate 7 and bearing the same reference numbers. Seating on the gasket 22 is a support ring 23 (FIG. 3(c)), the inner cylindrical face of which is grooved at 24 to receive an arcuate retaining spring 25 (FIG. 3(d)). The ring 23 also has holes corresponding to those of the base plate 7.

On the spring 25 rests a detector element 26 (FIG. 3(e)) of the form described with reference to FIG. 2 with its coated electrode surface uppermost. A further insulating gasket 27 (FIG. 3(f)) also of PTFE overlies the element 26 and ring 23, this gasket having a cut-out portion 28 located over the hole 19 in base plate 7. A metal gas distributor ring 30 (FIG. 3(g)) seats on the gasket 27 and has a series of tangentially directed ports 31 spaced around its inner periphery and connecting to an inlet disposed over the cut-out portion 28 of the gasket 27 so that gas entering the cell through the inlet connection 2 ows through the passageway defined by the bore 19 in the base plate 7 and the corresponding holes 12 in the gasket 22 and ring 23 and the cut-out portion 28, to be distributed around the periphery of the detecting element 26 by the ports 31 in the ring 3l). The latter ring also has an upstanding contact post 4 in the position corresponding to hole 19 in the elements beneath it, the metal of the ring 30 which presses against the outer peripheral contact area 32 of the electrode structure on detecting element 26 serving to form a conductive path between such contact area and the post 4. The lower portion of post 4 adjacent ring 30 is sheathed with insulating material.

A second insulating gasket 22 of the same form as shown in FIG. 3(1)) seats upon the upper surface of ring 30 with the post passing through a hole in the gasket corresponding to that referenced 19 in FIG. 3 (b).

A metal gas collector ring 33 FIG. 3(h) seats upon the second insulating gasket 22, this ring having a centrally recessed underface where it overlies the element 26 and being provided with a boss 34 at the centre of its recessed underface which serves the dual purpose of establishing electrical contact with the central contact area 37 of the detector element 26 and of providing outlet ports 35 connected through a radially extending passageway 36 in the material of the ring to` a bore corresponding to bore 20 in plate 7. Thus the gas admitted to the space above the detector element 26 is conducted away through ports 36 and passageway 36, the holes in the gaskets and rings underlying ring 33 and the bore 20 to the outlet connection 3. The ring 33 also has an upstanding contact post 5 in the position corresponding to the holes in register with bore 20 in plate 7. The lower portion of post 5 adjacent ring 33 is Vsheathed with insulating material. A third insulating gasket 22 of the form shown in FIG. 3r(b) overlies the collector ring 33I with the two contact posts 4 and v5 passing through holes corresponding to holes 19 and 20 in the gasket of FIG. 3(11) and the lassembly Iis completed by ya top plate 6 (FIG. 3(0) which `is a simpleI metal disc having six holes S to receive the six securing bolts 8 and two holes 4 and 5' to accommodate the contact posts 4 and 5.

With the various elements described in connection with FIG. 3 assembled and bolted together to form the cell of FIG. 1, gas introduced into the cell through the inlet connection 2 ows spirally across the face of the detector element 26 from the tangentially directed inlet ports 31 in distributor 30 to the central outlet ports 35 in the boss 34 of collector 33 and in so doing follows a path the area of which in contact with the hygroscopic film on element 26 diminishes in size progressively from the inlet ports to the outlet ports. This diminution in size follows the inverse square law and thus tends to balance the exponential decrease in moisture content of the gas which arises from pnogressive absorption of the moisture by the hygroscopic electrolyte film as the gas flows from the inlet ports to the outlet ports. As a result of this balancing effect the current density between electrodes of the two electrode patterns is substantially uniform over the whole active area of the detector element and there are thus no concentrations of current density to cause localised damage to the electrodes.

We claim:

1. A method of measuring the moisture content of a gas by causing the gas to ow through an electrolytic hygrometer cell to contact hygroscopic electrolyte material disposed between electrodes within such cell and measuring current flow between such electrodes, said method comprising causing the gas to flow in contact with said material along a spiral ow path of which the area in contact With said material progressively diminishes in size along the length thereof in the direction of liow.

2. The method according to claim 1 employing a cell having a central gas port of relatively small dimensions and at least one outer gas port of relatively large dimensions with said electrodes and material disposed between said central and outer ports, wherein said method comprises the further steps of causing the gas to flow from said outer gas port to said central gas port in contact with said material.

3. The method according to claim 1 wherein said method includes the step of causing said gas to flow spirally across the face of an electrically non-conductive substrate having at least two electrode structures embedded in such face and being coated with said hygroscopic electrolytic material, in a direction from the outer edge of said face towards the center thereof.

4. An electrolytic hygrometer cell for measuring the moisture content of a gas, said cell comprising a gas tight hollow housing, a detector element in the form of an electrically non-conductive substrate having at least two electrode structures embedded in one face thereof and with a coated of hygroscopic electrolytic material extending over said face disposed in the interior of said housing with said face in spaced relation to adjacent interior surfaces of said housing so as to permit gas to contact said coating, a gas inlet port in the periphery of said housing disposed to admit gas into the interior of said housing, said gas inlet port being directed substantially tangential- 1y with respect to said housing, a gas outlet port in the center portion of said housing spaced from said gas inlet port, said inlet and outlet ports, said interior surfaces of said housing and said coated face of said substrate dening between them a ow passage for .gas of which the area in contact with said coating progressively diminishes from said inlet port to said outlet por-t, and terminals extending from said electrode structures to the exterior of said housing.

5. A cell according to claim 4 wherein said gas inlet port is disposed adjacent an outer edge of said substrate and said gas outlet port is disposed at the center of said substrate.

6. A cell according to claim 4 wherein said substrate is a circular disc having said gas outlet port at the center thereof.

7. A cell according to claim 6 wherein said gas inlet port extends around the circumference of said disc.

8. A cell according to claim 6 wherein said substrate comprises a disc of glass having interdigitated patterns of grooves etched into a major face thereof, the grooves being lled with platinum to form said electrode structures.

9. A cell according to claim 8 wherein the outer sur` face of said electrode structures does not project above the adjacent outer surface of the glass disc.

10. A cell according to claim 8 wherein the platinum of said electrode structures is rhodium plated on the outer surface of such structures.

11. A detector element for an electrolytic hygrometer cell, said element comprising a flat disc substrate of electrically non-conductive material having two interdigitated electrode structures embedded in a major surface thereof, the outer surfaces of said electrode structures not projecting above the major surface of the substrate, and a coating of hygroscopic electrolytic material in contact with the outer surface of both embedded electrode structures 6 and the surface of the substrate intervening between adjacent parts of the respective electrode structures.

12. A detector element according to claim 11 wherein said substrate lis a glass disc having interdgitated patterns of grooves formed in a major face thereof, said grooves being filled with electrode material to form said electrode structures.

13. A detector element according to claim 12 wherein said grooves are filled with platinum.

14. A detector element according to claim 11 wherein each of said electrode structures comprises a plurality of concentrically arranged arcuate members interconnected in parallel, the interconnected members of one of said structures being connected to a first contact member disposed radially outwardly of said members and the interconnected members of the other of said structures being connected to a second contact member disposed radially inwardly of said structures.

15. A detector element according to claim 14 wherein said first contact member is of annular form and surrounds the interdigitated arcuate electrode members of both electrode structure.

16. A detector element according to claim 15 wherein said arcuate electrode members are arranged in four discrete quadrants.

References Cited UNITED STATES PATENTS 2,127,538 8/1938 Seiger 324-65 M 2,830,945 4/1958 Keidel 204-195 2,937,524 5/ 1960 Gregor 73-336.5 3,081,250 3/ 1963 Hall et al. 204-195 3,167,734 1/ 1965 Brucken et al. 73-3365 3,240,693 3/ 1966 Gardner 204-195 FOREIGN PATENTS 941,436 11/ 1963 Great Britain 204-195 TA-HSUNG TUNG, Primary Examiner U.S. Cl. X.R. 

